1. Field of the Invention
The present invention relates to a method of holding a plastic lens and a plastic lens holding structure.
2. Description of Related Art
In recent years, injection molded plastic lenses are widely used in optical instruments including, in particular, conventional photographic cameras, digital cameras equipped with solid state image forming devices, vide cameras, overhead projectors, film projectors and the like. Such a plastic lens is held in a lens holder forming a part of a lens barrel of an optical instrument with a retaining ring or adhered to the lens holder. In the case of using a retaining ring to hold the plastic lens in the lens holder, it is necessary to install a connecting mechanism such as a screw-connecting mechanism between the plastic lens and the lens holder. It is hard for current optical instruments that are confronted with a strong demand for miniaturization on to secure a space for the connecting mechanism. On the other hand, in the case of adhering the plastic lens to the lens holder, there is fear that the plastic lens is chemically corroded due to chemical reaction between an adhesive cement and an optical plastic material constituting the plastic lens or that an adhesive cement is sometimes hard to bond the plastic lens and the lens holder depending on adhesive cement and/or optical plastic material. Furthermore, it is hard to keep the plastic lens precisely held in the lens holder considering the effect of curing and shrinkage of the adhesive cement, a difference in thermal expansion coefficient between the plastic lens, the adhesive cement and the lens holder, and/or a change in ambient temperature.
There have been proposed techniques for riveting a plastic lens and a lens holder together as one of solutions to the problems. One of the techniques is disclosed, for example, in Japanese Unexamined Patent Publication No. 8-136783. This technique is such that a lens holder at its periphery is heated and deformed so as thereby to rivet or hot-weld the plastic lens to the lens holder and, in consequence, there is no necessity for a space for the connecting mechanism around the plastic lens, nor is there any necessity of using adhesive cement.
The prior art technique is, however, accompanied by the problem of thermal distortion of the plastic lens resulting from welding a periphery of the lens holder. Although it is of use to employ mechanical riveting in place of the hot riveting, the mechanical riveting subjects the lens holder to undesirably strong riveting force, possibly causing distortion of the plastic lens.